I brought a growler of my Milk Stout over to my buddies house, told him it was a milk stout, that is was made from milk sugar. He loved it. We killed the growler together.
Next morning, I get a call from his wife saying he's been puking his guts out all morning. I never knew he was lactose intolerant. I felt pretty bad, even though I did mention that it was a Milk Stout.
So just keep in mind, if you have a beer made with lactose and you're sharing it with people, ask them if they're lactose intolerant first!